This invention relates generally to magnetic tape transports, particularly to those suitable for use as or in a subsystem designed to enable a host system to obtain access to data on magnetic tape. More particularly, the invention relates to a method of, and apparatus for, reading data on the tape in such tape transports without being affected by dust that may accumulate on the tape or on the transducer.
Magnetic tape in cartridge form is being widely used as an external storage media of computer systems. Tape cartridges are now available in several different forms. Examples are the two-reel, belt driven cartridge introduced by Minnesota Mining and Manufacturing Company, and the "digital cassette" based on the standard audio cassette developed by Philips and made to similar dimensions though with more precision.
Such tape cartridges include a length of magnetic tape with a plurality or multiplicity of parallel tracks extending longitudinally of the tape. The tape is bidirectional, so that the tracks consist of forward tracks, on which data is written and read during tape travel in a predetermined forward direction, and reverse tracks. A stream of data is recorded on each track in the form of blocks for convenience of handling and particularly of error recovery. Each data block includes a block marker, recorded as a predefined sequence of reversals of magnetization, which indicates the presence of a data block there on the tape.
Tape transports for use with such tape cartridges include a blank detector circuit for detecting the end of the recorded data on each track of the tape. The blank detector circuit relies on the block markers for detecting a predetermined stretch of blank track, determining that the data end has been reached on the track when no block marker has been included in the output from the transducer for a preassigned length of time. Inputting this output from the blank detector circuit, the controller of the tape transport will terminate reading on the track.
A problem has been encountered in detecting the end of data blocks on each tape track as above, particularly in cases where the tape transports are used in a dusty environment. Dust on the tape or the heads can prevent the correct reading of the recorded information. If the transducer fails to read the block markers, the blank detector circuit has so far erroneously informed the controller that there is no more data on the track, even though actually there are still more data blocks left on the same track.
Usually, in bidirectional tape transports, the tape is alternately read forwardly and reversely. Such alternate bidirectional reading of the tape results in the accumulation of dust on its opposite end portions as the tape slides past the heads. Consequently, if the accumulated dust on either end portion of the tape prevents the reading of the block markers on any track, the controller has so far erroneously determined that there is no more data, or no data at all, on the track immediately after the commencement of reading on that track.
A wrong detection of data end is objectionable for two reasons. First, it makes impossible the reading of the remaining data on each tape track. Second, new data may be overwritten on the remaining data blocks with the consequent erasure of the old data.